This invention relates to a novel and useful graft copolymer. More particularly, it relates to a graft copolymer having dispersion stabilizing effect, a process for producing it and a process of emulsion polymerization of vinyl monomers using the graft copolymer as a dispersion stabilizer.
In emulsion polymerization, monomers incorporated in micelles of an emulsifier (surface active agent) are polymerized into polymer particles. These particles generally have small diameters ranging from 0.05 .mu. to 1 .mu., so that it is difficult to remove the emulsifier from the polymer produced. Therefore, there has been developed an emulsifier-free emulsion polymerization method which uses no surface active agent. In this method, polymerization is in principle carried out with only a polymerization initiator without any dispersion stabilizer, or polymerization is carried out by preliminarily polymerizing or emulsion polymerizing a water-soluble monomer thus forming into an easily water-soluble or water-dispersible dispersion. Alternately, polymerization is carried out by previously adding a highly water-soluble or water-dispersible dispersion stabilizer or a water-soluble polymer compound. The dispersion stabilizer used in this case comprises a hydrophobic carbon backbone and a side chain having hydrophilic groups and is classified as a linear polymer on the basis of its polymer structure.
According to the emulsifier-free emulsion polymerization method with the use of this dispersion stabilizer, a dispersion having a uniform particle size in the range of ca. 0.1.about.0.5 .mu. is obtained.
In the aforementioned emulsifier-free emulsion polymerization method using the dispersion stabilizer, however, preparation of a dispersion containing polymer particles having a particle size of 0.5 .mu. or more will result in the distribution of the particle diameter over a wide range or the agglomeration of the polymer particles forming into a large amount of agglomerate, which makes it difficult to conduct stable emulsion polymerization.